MOBILITY ENHANCEMENT IN SiGe HETEROJUNCTION BIPOLAR TRANSISTORS

ABSTRACT

The present invention relates to a high performance heterojunction bipolar transistor (HBT) having a base region with a SiGe-containing layer therein. The SiGe-containing layer is not more than about 100 nm thick and has a predetermined critical germanium content. The SiGe-containing layer further has an average germanium content of not less than about 80% of the predetermined critical germanium content. The present invention also relates to a method for enhancing carrier mobility in a HBT having a SiGe-containing base layer, by uniformly increasing germanium content in the base layer so that the average germanium content therein is not less than 80% of a critical germanium content, which is calculated based on the thickness of the base layer, provided that the base layer is not more than 100 nm thick.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/212,187, filed Aug. 26, 2005.

FIELD OF THE INVENTION

The present invention relates to a SiGe-based heterojunction bipolartransistor, and more particularly, to a SiGe-based heterojunctionbipolar transistor that has improved performance due to mobilityenhancement. The present invention is also related to a method offabricating such a SiGe-based heterojunction bipolar transistor.

BACKGROUND OF THE INVENTION

In the state-of-the-art SiGe heterojunction bipolar transistor (HBT)devices, the base material is epitaxdially deposited by means ofchemical vapor deposition (CVD) or molecular beam epitaxy (MBE) asfront-end-of-line (FEOL) films relatively early in the manufacturingprocess. This offers the possibility of tailoring specific base profilesin both alloy and dopant and allows pseudomorphic growth of alloys ofsilicon with germanium and carbon, which can be used to improveperformance of the HBT devices.

Specifically, incorporation of substitutional germanium into the crystallattice of the silicon creates a compressive strain in the material,because the Ge atom requires a larger atomic separation. It also reducesthe bandgap of the material. In some SiGe-based heterojunction bipolartransistor (HBT) devices, the Ge content increases abruptly to aconstant value across the entire base region (single rectangularprofile) or parts thereof (stepped profile). In a “graded” SiGe HBTdevice, the Ge content in the base region is not a constant, but insteadincreases from a low Ge concentration near the emitter-base junction toa high Ge concentration deeper into the base region, thus creating adrift field with decreasing bandgap in the direction of the electronflow. The electrons injected from the emitter of the HBT device face areduced injection barrier due to the low Ge concentration at theemitter-base junction, and then experience an accelerating field acrossthe base region due to the increasing Ge content deeper into the baseregion. The low Ge content at the emitter-base junction increases theelectron injection into the base, thus increasing the current gain. TheGe grading in the base region has the effect of speeding the transportof electrons across the device, resulting in reduced transit timethrough the base, which is of particular importance in scaling thedevice to a higher-speed performance. Such a desired Ge grading can bereadily created by time-dependent programming of the Ge precursor flowsduring the SiGe film deposition.

However, when the strain in the pseudomorphically grown SiGe filmreaches a critical level, either due to increase of the SiGe filmthickness or increase of the Ge content, it can no longer be containedby elastic energy stored in the distorted SiGe crystal structure.Instead, a portion of the strain will be relaxed through generation ofmisfit dislocations in the heteroepitaxial interface. Therefore, for aSiGe film of a specific Ge content, there exists a “critical thickness,”defined as the maximum thickness for the pseudomorphic growth of theSiGe film, below which the strain caused by lattice mismatch between Siand Ge is contained by elastic energy stored in crystal latticedistortion, and above which a portion of the strain is relaxed throughgeneration of misfit dislocations in the heteroepitaxial interface.Similarly, for a SiGe film of a specific thickness, there exists a“critical Ge content,” which is defined as the maximum germanium contentthat can be incorporated into the pseudomorphic SiGe film, below whichthe strain caused by lattice mismatch between Si and Ge is contained byelastic energy stored in crystal lattice distortion, and above which aportion of the strain is relaxed through generation of misfitdislocations in the heteroepitaxial interface.

Dislocation defects originated from strain relaxation are electricallyactive and can cause increased carrier scattering, carrier trapping, andcarrier recombination. Therefore, in the past, the Ge content and totalthickness of a SiGe base layer were carefully designed not to exceed thecritical values, in order to avoid formation of dislocation defects inthe device structure.

Recent aggressive scaling of the SiGe HBT devices in both the verticaland lateral directions has led to significant reductions in devicedimensions, including significant reduction in the base layer thickness.Further, recent high-frequency measurements indicate that carrierstraveling through ultra-thin base layers of high-performance HBTs (e.g.,having a thickness of not more than about 100 nm) have already reached asaturation velocity at the today's aggressive Ge grading. In otherwords, increased Ge grading in the ultra-thin base layers does not yieldfurther improvements in carrier velocity.

As a result, state-of-the-art SiGe-based HBT devices (see Khater et al.,“SiGe HBT Technology with fMax/fT=350/300 GHz and Gate Delay Below 3.3ps,” IEEE Electron Devices Meeting, IEDM Technical Digest, 13-15 Dec.2004, pp. 247-250) have base layers with Ge content and thickness thatare well below the critical values.

SUMMARY OF THE INVENTION

The present invention seeks to further improve performance of currentlyavailable SiGe-based HBT devices by increasing biaxial strain in thebase region of the HBT devices, which in turn increases carrier mobilityin the base region.

The present invention discovers that although a further increase in theGe content of the ultra-thin base layers of the currently availableSiGe-based HBT devices does not further increase carrier velocity, itcan cause increase in biaxial strains near the base region, i.e.,increased compressive strain along the direction parallel to thesubstrate surface (i.e., the lateral direction) and increased tensilestrain along the direction perpendicular to the substrate surface (i.e.,the vertical direction), which functions to enhance mobility of holeslaterally flowing through the base region and electrons verticallytraversing the base region.

Since the carrier base-transit time depends not only on carriervelocity, but also on carrier mobility, the carrier base-transit time ofthe currently available SiGe-based HBT devices can be further reduced byincreasing the Ge content of the ultra-thin base layers of such HBTdevices to near-critical value.

Further, the base resistance of the SiGe-based HBT devices also dependson the carrier mobility, so an increase of the base layer Ge content tonear-critical value can also be used to reduce the base resistance.

In one aspect, the present invention relates to a HBT device containinga collector region, a base region, an extrinsic base region, and anemitter region. The base region of the HBT device comprises anultra-thin SiGe-containing layer, i.e., having a thickness of not morethan about 100 nm. A critical germanium content can be predetermined forsuch an ultra-thin SiGe-containing layer, based on its thickness, andthe SiGe-containing layer is arranged and constructed so that it has agermanium content profile with an average germanium content of not lessthan about 80% of the predetermined critical germanium content.

Preferably, the average germanium content in the ultra-thinSiGe-containing layer is not less than 90%, more preferably not lessthan 95%, and still more preferably not less than 99%, of thepredetermined critical germanium content. Most preferably, the averagegermanium content in the ultra-thin SiGe-containing layer issubstantially equal to (i.e., with ±0.1% difference) the predeterminedcritical germanium content.

Critical germanium content for the ultra-thin SiGe-containing layer canbe readily calculated by various conventionally known methods, asdescribed hereinafter in greater detail, and the present inventionselects the average calculated critical germanium content forcontrolling the actual germanium content in the SiGe-containing layer,so as to minimize the risk of dislocation generation. For example, for aSiGe-containing layer of about 50 nm thick, the calculated criticalgermanium content is between about 16 atomic % to about 18 atomic %,while the average value of 17 atomic % is selected as the predeterminedcritical germanium content in the present invention. For anotherexample, the calculated critical germanium content of a 100 nm thickSiGe-containing layer is between about 9 atomic % to about 11 atomic %,and the average value of 10 atomic % is selected as the predeterminedcritical germanium content for practice of the present invention.

The ultra-thin SiGe-containing layer of the present invention may have aflat Ge content profile (i.e., a substantially uniform Ge content isprovided across the entire SiGe-containing layer), a multi-step Gecontent profile (i.e., multiple plateaus of uniform Ge content areprovided across the entire SiGe-containing layer), or a graded Gecontent profile (i.e., the Ge content changes in the SiGe-containinglayer). The term “Ge content profile” or “germanium content profile” asused herein refers to a plot of germanium contents in a structure as afunction of thickness or depth in the structure. Preferably, theultra-thin SiGe-containing layer has a graded Ge content profile, whichmay have any suitable shape, either regular or irregular. For example,such an ultra-thin SiGe-containing layer may have a triangular Gecontent profile, or a trapezoidal Ge content profile.

For a simple (i.e., stepped) or complicated (graded) SiGe-containinglayer, the “average Ge content” is determined by first integrating theGe content over the entire SiGe-containing layer, i.e., so as todetermine the total or integrated Ge content in the layer, and thendividing the integrated Ge content over the thickness of the layer. ASiGe-based HBT is found to be stable in further high-temperatureprocessing steps, which are required to finish the HBT device, as longas the average Ge content in the base layer of such a SiGe-based HBTdevice remains below or equal to a critical Ge content corresponding tothe thickness of the base layer. The critical Ge content can be readilydetermined, for example, from the Matthew/Blakeslee line (MBL) that isto be described in greater detail hereinafter. Moreover, certaindeposition techniques, such as ultra-high vacuum chemical vapordeposition (UHVCVD) and high-temperature bake-conditioned remoteplasma-enhanced chemical vapor deposition (RPCVD), allow theSiGe-containing base layer to be deposited with an average Ge contentthat is very close (more than 95%) to the critical Ge content.

In another aspect, the present invention relates to a heterojunctionbipolar transistor that comprises a SiGe-containing base layer having athickness of not more than about 50 nm and a germanium content profilewith an average germanium content ranging from about 16.5 atomic % toabout 17.5 atomic %.

In a further aspect, the present invention relates to a method forenhancing carrier mobility in a heterojunction bipolar transistor thathas an ultra-thin SiGe-containing base layer, without changingquasi-static drift field of the base layer. The quasi-static drift fieldof a SiGe-containing layer depends on the Ge grading rate or the shapeof the Ge content profile, but not the absolute Ge content, in theSiGe-containing layer.

Therefore, a uniform increase in the Ge content across the ultra-thinSiGe-containing base layer can be used to reach near-critical Ge contentin the base layer, thereby maximizing the biaxial strain and the carriermobility in the base layer, but it does not change the Ge grading rateor the shape of the Ge content profile and thus maintains the samequasi-static drift field in the base layer.

In one embodiment, the method of the present invention comprises:

-   -   measuring the thickness of the SiGe-containing base layer;    -   calculating a critical germanium content based on the thickness        of the SiGe-containing base layer;    -   measuring germanium content in the SiGe-containing base layer to        determine the germanium content profile of said SiGe-containing        base layer; and    -   changing the germanium content profile of the SiGe-containing        base layer, by uniformly increasing the germanium content in the        SiGe-containing base layer by a sufficient amount so that the        changed germanium content profile has an average germanium        content of not less than about 80% of the calculated critical        germanium content.

In a still further aspect, the present invention provides a method forfabricating a high performance SiGe-based HBT device, by:

-   -   determining a projected thickness and a projected germanium        profile for a SiGe-containing base layer of the SiGe-based HBT        device, wherein said projected thickness is not more than about        100 nm;    -   calculating a critical germanium content based on the projected        thickness and an average germanium content based on the        projected germanium profile and the critical germanium content,        wherein said average germanium content is not less than 80% of        the critical germanium content;    -   forming a collector for the HBT device in a semiconductor        substrate;    -   depositing over the collector a SiGe-containing base layer,        which has the projected thickness, the projected germanium        profile, and the calculated average germanium content; and    -   forming an extrinsic base and an emitter for the HBT device.

The projected thickness and the projected germanium profile can bereadily determined by theoretical band-structure calculations andhistorical base profile scaling, which are known in the art andtherefore are not described in detail herein. Preferably, the projectedgermanium profile provides for germanium grading over the base layer,which establishes a quasi-static drift field for accelerating carriersacross the base layer.

Other aspects, features and advantages of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of an exemplary prior art SiGe-basedHBT device.

FIG. 2 shows two prior art base Ge content profiles for SiGe-based HBTdevices.

FIG. 3 shows an improved base Ge content profile for a high performanceSiGe HBT device, in comparison with a prior art base Ge content profile,according to one embodiment of the present invention.

FIG. 4 shows another improved base Ge content profile for a highperformance SiGe HBT device, in comparison with a prior art base Gecontent profile, according to one embodiment of the present invention.

FIG. 5 shows yet another improved base Ge content profile for a highperformance SiGe HBT device, in comparison with a prior art base Gecontent profile, according to one embodiment of the present invention.

FIG. 6 shows a Matthews-Blakeslee curve that can be used for determiningthe critical Ge content for a SiGe-containing layer based on itsthickness.

DETAILED DESCRIPTIONS OF THE INVENTION, AND PREFERRED EMBODIMENTSTHEREOF

A typical SiGe-based HBT having a deep trench isolation and a T-shapedemitter is shown in FIG. 1 (hereinafter FIG. 1). Specifically, FIG. 1includes a semiconductor structure 10 that includes at least a collector12 that is positioned between two shallow trench isolations regions 14Rand 14L. The shallow trench isolation region on the left hand side ofthe drawing, represented by 14L, has a deep trench 16 extending from abottom walled surface of the shallow trench. The semiconductor structureshown in FIG. 1 also includes a first epitaxial silicon layer 18, a SiGebase 20, and a second epitaxial silicon layer 22, which are located atopthe trench isolation regions and the collector 12. The structure shownin FIG. 1 also includes an extrinsic base 24 and an oxide layer 26 whichare patterned to expose a surface of the second epitaxial silicon layer22 that is located above the SiGe base 20. Nitride spacers 28 arelocated on sidewalls of the oxide layer 26 and the extrinsic base 24. AT-shaped emitter 30 is present in the structure as shown in FIG. 1.

The HBT shown in FIG. 1A is fabricated utilizing conventional bipolarprocessing techniques that are well known in the art. For example, aheterojunction Si-containing base, particularly comprised of SiGe, isepitaxially grown on a collector pedestal surrounded by isolation oxide.

During epitaxial growth, sophisticated boron, germanium, and carboncontent profiles (with either exponential or polynomial ramps) canreadily be created by time dependent programming of precursor flows.Graded germanium content profiles are desirable for creating built-indrift fields that accelerate carriers across the otherwise neutral baseregion of the transistor, thus drastically reducing the transit time.

Although germanium content profiles and germanium grading inconventional SiGe-based HBT devices were used to be limited by thecritical thickness and critical germanium content of the SiGe baselayer, recent aggressive scaling of the SiGe HBT devices has led tosignificant reductions in device dimensions, including significantreduction in the base layer thickness. Further, since recent studiesindicate that carriers traveling through ultra-thin base layers havealready reached a saturation velocity at the today's moderate Ge gradingand that increased Ge grading in the ultra-thin base layers does notyield further improvements in carrier velocity, currently availableSiGe-based HBT devices have base Ge content well below the criticalvalue.

FIG. 2 shows two exemplary graded Ge profiles in currently availableSiGe-based HBT devices. The average Ge contents for these two graded Geprofiles (x_(A1) and x_(A2), respectively) are well below the criticalGe content (x_(c)) of the ultra-thin SiGe base layers of such devices.

The present invention discovers that although further increase in the Gecontent of the ultra-thin base layers of the currently availableSiGe-based HBT devices does not further increase carrier velocity, itcan cause increase in biaxial strains near the base region, therebyenhancing carrier mobility in the base region and reducing the carrierbase-transit time as well as the base resistance in the SiGe-based HBTdevices.

Therefore, the present invention utilizes near-critical average Gecontent in the ultra-thin base region of a SiGe HBT, so as to increasecarrier mobility and further reduce base resistance and carrier transittime through the neutral base region. The method described by thepresent invention can be used to modify and improve the performance ofan existing SiGe HBT device, or to fabricate a high performance SiGe HBTdevice ab initio.

In order to maintain the same drift fields created by the graded Gecontent profiles in the ultra-thin base region of the existing SiGe HBT,the present invention proposes modification of the existing SiGe HBT, byuniformly increasing the germanium content in the SiGe base layer of theexisting HBT device by a sufficient amount so that the average germaniumcontent of the SiGe base layer is close to, or at least near, 80% of thecritical germanium content.

FIG. 3 shows a graded Ge content profile 14, which is created byuniformly increasing Ge content in the graded Ge content profile 12 ofan existing SiGe HBT device having an ultra-thin base region, accordingto one embodiment of the present invention. The increased Ge content isreferred to as Δx, and the average Ge content (x_(A)) in the new gradedGe content profile 14 is significantly closer to the critical germaniumcontent (x_(c)) than the average Ge content (not shown) in the prior artGe content profile 12.

Similarly, FIG. 4 shows a graded Ge content profile 24, which is createdby uniformly increasing Ge content (by Δx) in the prior art graded Gecontent profile 22 of an existing SiGe HBT device having an ultra-thinbase region, according to one embodiment of the present invention. Theaverage Ge content (x_(A)) in the new graded Ge content profile 14 issignificantly closer to the critical germanium content (x_(c)) than theaverage Ge content (not shown) in the prior art Ge content profile 12.

FIG. 5 shows another graded Ge content profile 34, which is created byuniformly increasing Ge content by Δx in both the ultra-thin SiGe baseregion of an existing SiGe HBT and the two epitaxial silicon layersflanking the ultra-thin SiGe base (i.e., layers 18 and 22 of FIG. 1),according to another embodiment of the present invention. The Ge contentincreases in the two epitaxial silicon layers are indicated by ramps 36a and 36 b in FIG. 5, and the average Ge content (x_(A)) in the newgraded Ge content profile 34 is significantly closer to the criticalgermanium content (x_(c)) than the average Ge content (not shown) in theprior art Ge content profile 32.

Therefore, the increase in Ge content can be either limited to only theultra-thin SiGe base region, so that the epitaxial silicon layersflanking such an ultra-thin SiGe base consist essentially of silicon,with little or no Ge therein, or it can be extended to also the flankingepitaxial silicon layers, forming an extended SiGe epitaxial baseregion.

The present invention provides a method to enhance the carrier mobilityin a SiGe-based HBT device while reducing the base resistance of thetransistor. In accordance with the present invention, carrier mobilityenhancement is achieved by changing the Ge profile in the ultra-thinbase region of the HBT device, without negatively impacting the driftfields that are typically associated with bipolar transistors.

More particularly, the present invention provides a method in which theGe content profile in the ultra-thin base region of a SiGe HBT device ischanged to provide the simultaneous application of lateral compressiveand vertical tensile strain. This change in Ge content profile asdescribed by the present invention does not negatively affect, orsignificantly alter, the quasi-static drift field created by the amountof Ge grading in the ultra-thin base region. By adding a uniform amountof additional Ge to the base Ge content profile and increasing theaverage Ge content in the ultra-thin base layer to near-critical value,the internal biaxial layer strain can be greatly enhanced up to theapparent and metastable critical point of relaxation.

The critical Ge content for a SiGe base layer of a specific thicknesscan be readily determined by various methods, as described by J. C. Beanet al., “Ge_(x)Si_(1-x)/Si Strained-Layer Superlattice Grown byMolecular Beam Epitaxy,” J. VAC. SCI. TECHNOL., Vol. A2, No. 2, pp.436-440 (1984); J. H. van der Merwe, “Crystal Interfaces. Part I.Semi-Infinite Crystals,” J. APPL. PHYS., Vol. 34, No. 1, pp. 117-122(1963); J. M. Matthews and A. E. Blakeslee, “Defects in EpitaxialMultilayers I. Misfit Dislocations in Layers,” J. CRYSTAL GROWTH, Vol.27, pp. 118-125 (1974); S. S. Iyer et al., “Heterojunction BipolarTransistors Using Si—Ge Alloys,” IEEE TRANSACTIONS ON ELECTRON DEVICES,Vol. 36, No. 10 (October 1989); R. H. M. van der Leur et al., “CriticalThickness for Pseudomorphic Growth of Si/Ge Alloys and Superlattice,” J.APPL. PHYS., Vol. 64, No. 5, pp. 3043-3050 (15 Sep. 1988); and D. C.Houghton et al., “Equilibrium Critical Thickness for Si_(1-x)Ge_(x)Strained Layers on (100) Si,” APPL. PHYS. LETT., Vol. 56, No. 5, pp.460-462 (29 Jan. 1990).

FIG. 6 shows a Matthews-Blakeslee curve that correlates the criticalthickness of a SiGe-containing film with the Ge content therein, whichcan be readily used to determine the critical Ge content giving aspecific thickness of the SiGe film.

The critical Ge contents calculated by using different methods maydiffer slightly from one another, due to the different models used anddifferent parameters considered. The present invention selects theaverage calculated critical germanium content for controlling the actualgermanium content in the SiGe-containing layer. For example, for aSiGe-containing layer of about 50 nm thick, the calculated criticalgermanium content is between about 16 atomic % to about 18 atomic %,while the value of 17 atomic % is selected as the predetermined criticalgermanium content in the present invention. For another example, thecalculated critical germanium content of a 100 nm thick SiGe-containinglayer is between about 9 atomic % to about 11 atomic %, and the value of10 atomic % is selected as the predetermined critical germanium contentfor practice of the present invention.

Preferably, the ultra-thin SiGe base layer with the near-critical Gecontent is pseudomorphically grown by chemical vapor deposition (CVD),with well-established process control and proven repeatability andsuitable for batch processing and large-scale manufacturing. Inaddition, CVD process requires no plasma treatment, and thesubstitutional Ge atoms are electrically inactive, save for minutechanges in band structure and ensuring ultra-low contamination levels inthe base layers.

While the present invention has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present invention. It is therefore intended that the presentinvention not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A heterojunction bipolar transistor comprising a collector region, abase region, an extrinsic base region, and an emitter region, whereinthe base region comprises a SiGe-containing layer, wherein theSiGe-containing layer has a thickness of not more than about 100 nm anda predetermined critical germanium content associated with saidthickness, and wherein the SiGe-containing layer has a germanium contentprofile with an average germanium content of not less than about 80% ofthe predetermined critical germanium content.
 2. The heterojunctionbipolar transistor of claim 1, wherein the germanium content profile ofthe SiGe-containing layer is stepped or graded, and wherein the averagegermanium content in the SiGe-containing layer is determined byintegrating germanium content over the entire SiGe-containing layer, soas to determine an integrated germanium content in the layer, anddividing the integrated germanium content over the thickness of saidlayer.
 3. The heterojunction bipolar transistor of claim 1, wherein theaverage germanium content in the SiGe-containing layer is not less thanabout 90% of the predetermined critical germanium content.
 4. Theheterojunction bipolar transistor of claim 1, wherein the averagegermanium content in the SiGe-containing layer is not less than about95% of the predetermined critical germanium content.
 5. Theheterojunction bipolar transistor of claim 1, wherein the averagegermanium content in the SiGe-containing layer is not less than about99% of the predetermined critical germanium content.
 6. Theheterojunction bipolar transistor of claim 1, wherein the averagegermanium content in the SiGe-containing layer is substantially equal tothe predetermined critical germanium content.
 7. The heterojunctionbipolar transistor of claim 1, wherein the predetermined criticalgermanium content of the SiGe-containing layer is not less than about 10atomic %.
 8. The heterojunction bipolar transistor of claim 1, whereinthe SiGe-containing layer having a thickness of not more than about 50nm, and wherein the predetermined critical germanium content of theSiGe-containing layer is not less than about 17 atomic %.
 9. Theheterojunction bipolar transistor of claim 1, wherein the base regioncomprises two epitaxial semiconductor layers, and wherein theSiGe-containing layer is sandwiched between said two epitaxialsemiconductor layers.
 10. The heterojunction bipolar transistor of claim9, wherein the two epitaxial semiconductor layers both consistsessentially of silicon.
 11. A heterojunction bipolar transistorcomprising a SiGe-containing base layer having a thickness of not morethan about 50 nm and a germanium content profile with an averagegermanium content ranging from about 16.5 atomic % to about 17.5 atomic%.